Because San Francisco is surrounded on three sides by water, temperatures do not drastically change between seasons. Winter sees an average rainfall of 23 inches with temperatures ranging from 45-57 degrees. Summer’s temperatures range from 50-70 degrees. What keeps the City cool during the summer months is the offshore fog[1]. Hot inland temperatures create a low pressure zone over California’s Central Valley 90 miles away. As hot air rises, heavier cold ocean air is pulled inland, typically shrouding the western half of the City in fog. Hills located in the center of the City block the fog from getting to most of the eastern Districts. Fog typically burns off around 10:00am but returns in the late afternoon. September and October are the City’s warmest months: the air is clear, the sun is out, and temperatures can get above 70 degrees.
Tracking San Francisco weather began on August 14, 1849 by a resident named Thomas Tennent. Tennent would continue keeping detailed records until March 1, 1871 when the US Army Signal Service took over. The earliest recorded snowfall was on December 20, 1856, when 2.5 inches of snow blanketed the City. Since then there have only been nine other snowfalls[2]. The heaviest was in 1887 when 3.7 inches fell in downtown, with the higher elevations in the western part of the City getting up to 7 inches.
The coldest day on record was on December 11, 1932 at 27 degrees. The hottest day was on September 1, 2017 at 106 degrees. There have only been six days since 1921 that reached over 100 degrees[3]. The heaviest rainfall occured in 2005 when 5 inches of rain fell over a 24 hour period.
In 2008 a weather beacon was installed on the crown of the South Tower of One Rincon Hill, an upscale residential complex consisting of two 50 and 60-story towers respectively. The beacon changes color based on the upcoming forecast: red indicates warmer weather, blue indicates colder weather, green signifies rain and yellow means no change.
[1]Karl the Fog: https://thesanfranciscophoenix.com/?p=3950
[2] 1856, 1868, 1882, 1884, 1887, 1888, 1951, 1962, 1976 and 2011
[3] 1932, 1971, 1984, 1987, 2000 and 2017
